This invention relates generally to the field of talc and more specifically relates to the use of a treated talc in paper products.
In the field of offset printing, the ink is first transferred from the printing plate to a blanket and then from the blanket to paper. This offset mode is also used commercially for printing letterpress, called dry offset, or gravure, called offset gravure. In offset printing, some picking of fibers or fillers from the paper surface, called linting or dusting, is normal due to some stiff, unfibrillated fibers that are not completely bonded to the sheet. Offset linting or dusting is a sheet surface phenomenon. However, these fibers and fillers because they are loosely bound to the paper can be transferred to the blanket, which will cause printing imperfections in resultant copies and inferior print copy. In addition, excessive linting or dusting will also lead to increased blanket maintenance. Thus, offset printers desire to use paper which does not lint or dust excessively.
In the paper-making process, cellulosic fibers are softened with water before being processed into paper. Fillers such as clay, titanium dioxide, talc, and calcium carbonate, are added to the papermaking process to improve paper properties such as opacity, brightness, and printability. Each filler is unique due to differences in physico-chemical and morphological properties. One example of a substance which will reduce linting or dusting is clay. Titanium dioxide is an excellent filler for opacity purposes due to is high refractive index and particle size. Due to its hydrophobicity, talc is an excellent pitch/stickie control. In addition, talc is an excellent filler for purposes of improving machine drainage, sheet smoothness and printability. However, as a filler, talc does not affiliate with the water-based cellulosic fibers as well as certain other fillers that are more hydrophilic. Thus, a method of modifying talc""s affinity to the cellulosic fibers of the raw paper is needed.
This invention discloses a sized and treated talc composition that is useful in preventing dusting or linting of paper when such talc is added to paper pulp during the paper making process. Also disclosed is a method of preparation of the talc of the present invention and its use in making paper. More particularly, the talc of the present invention is milled to have a particle size of less than 10 micrometers and a cationic charge to the surface of the talc particles. For example, a cationic charge can be added to the particle by mixing talc particles with water to create a slurry and, adding a cationic compound to the slurry. In preferred embodiments, the cationic compound is selected from cationic wet-end starch, cationic wax-based emulsion, polydadmacs and carboxymethylcellulose. Also, preferably, the talc particles have a top size of less than about 1.5% and a particle size of about 5 microns.
The present invention is also directed to a paper product which includes the talc composition of the present invention and a method for making such a paper product. The method includes adding the talc additive to a paper pulp and forming the paper pulp into a paper product.
The present invention is directed to a treated talc composition, a method of making the same and use of the composition as a filler in making paper. The use of untreated talc in the paper making process is well known and has been performed for many years. However, the use of untreated talc results in linting and dusting problems. Talc is more prone to linting or dusting than clay or other fillers due to talc""s hydrophobicity and chemically inert planar surface. Some paper making mills add a strengthener, such as starch, to the paper pulp mixture in order to improve the surface strength of the paper and overcome linting. However, the addition of such a strengthener can affect the cost and the processing of the paper pulp. The process of the present invention for making the treated talc composition includes processing talc to a talc particle size of less then 10 micrometers in diameter, and imparting a cationic charge to the surface of the talc particles.
Talc, in its unadulterated form, is a platy mineral, meaning that talc will crystallize in a thin sheet and will tend to flake along cleavage planes. Talc can be processed as part of the present invention by any suitable method. For example, one such method is by milling talc with an air classified mill (xe2x80x9cACMxe2x80x9d). Here, the talc is dry and is hammered to achieve a desired particle size distribution curve. The talc particles are then screened to the desired median particle size. The ACM process produces more fractured talc particles than delaminated talc particles. Another talc milling method is a fluidized energy method (xe2x80x9cFEMxe2x80x9d). Here, the talc is mixed into a slurry with water such that the talc is held in suspension and the talc particles are sorted to achieve the right particle size distribution curve. Although both methods will produce talc that is suitable for use in the present invention, talc produced by ACM processing is preferred because it produces a talc material with a lower top size (i.e., the quantity of particles retained on a 325-mesh screen or greater than 45 microns in diameter). The advantages of a smaller top size are discussed in more detail below.
A cationic charge is imparted to the surface of the talc particles. In this manner, it is believed that the ability of the talc to adhere to the paper surface is improved to achieve reduced dusting and/or linting during offset printing. In preferred embodiments, the cationic charge on the talc particles is retained during reslurrying, such as during a paper making process, and more preferably, the cationic charge on the talc particles is retained in the presence of anionic trash (filtrate from a groundwood pulp). The step of imparting a cationic charge to the surface of the talc particles can be accomplished by mixing the talc particles with water to create a slurry and, adding a cationic compound to the slurry. The cationic compound can be any suitable cationic compound and in particular, can be cationic wet-end starch, cationic wax-based emulsion, polydadmacs and carboxymethylcellulose.
In another aspect of the invention, the talc particles in the talc composition have a top size of less than about 1.5%, more preferably less than about 1.0%, and even more preferably, less than about 0.5%. As noted above, the term top size refers to the weight percentage of the talc particles in a given composition which are retained on a 325-mesh screen or are greater than 45 microns in diameter. In preferred embodiments, talc compositions in accordance with the top size parameters of the present invention are prepared by milling in an air classified mill process. As demonstrated in the Examples, it has been found that use of talc compositions in accordance with the top size parameters of the present invention significantly reduces linting and dusting during offset printing.
As noted above, the process of the present invention includes processing talc to a talc particle size of less then 10 micrometers in diameter. In a further aspect of the present invention, the talc particles in the talc composition can have a median particle size of about 7 microns, and in a further embodiment, can have a median particle size of about 5 microns. It has been found that particle size of the talc composition has varying effects on its use as a filler in making paper. For example, as demonstrated in the Examples, larger particle sizes, such as 10 microns and 7 microns, produce faster drainage of liquid during paper production. It is believed this effect on drainage is achieved because the larger particle sizes open up the wet web during sheet forming allowing faster drainage. Alternatively, smaller particle sizes, such as 5 microns provide porosity and sheet gloss benefits.
The present invention is also directed to a method of making a paper product which includes adding a talc additive of the present invention to a paper pulp and forming the paper pulp into a paper product. In any given paper making process, various stages are set for additives to be mixed with the cellulose fibers that make up paper pulp. When talc is used as an additive to the paper, the talc is processed through three stages before reaching the paper pulp. The talc is prepared in a unit which is typically called a talc dispersion unit. The talc is mechanically sheared in the talc dispersion unit. From the talc dispersion unit, the talc enters into a storage chest. From the storage chest, the talc is processed through a talc delivery line where it is finally added to the paper pulp. According to the present invention, the talc additive can be prepared by imparting a cationic charge to the surface of the talc particles at any of the three stages, either at the talc dispersion unit, the talc storage chest, or in the talc delivery pipe. Further, the steps of adding a talc additive to a paper pulp and forming the pulp into a paper product are conducted according to conventional processes.